1. Field of the Invention
The present invention relates to a driving apparatus and equipment comprising the driving apparatus.
The present invention relates to a driving apparatus and equipment for moving a member e.g. the lens of a camera by means of a plurality of electro-mechanical transducer such as piezoelectric elements.
2. Description of the Prior Art
A description will be given to a conventional driving apparatus which performs linear driving by means of piezoelectric elements with reference to FIG. 11 showing a lens unit for driving a plurality of lenses. FIG. 11 is a perspective view of the lens unit.
201, 205 denote lens barrels for holding lenses. Shafts 202, 203 for slidably guiding the lens barrels 201, 225 in the direction of an optical axis extend through projections 201a, 201c, 225a, 225c formed integrally with the lens barrels 201, 225. The shafts 202, 203 have their respective near-front and near-rear end portions held slidably lengthwise thereof in respective holes formed in the upright portions 213a, 218a of support members 213, 218 and in respective holes formed in the upright portions 213c, 218c of the support members 213, 218.
The projections 201a, 201c, 225a, 225c are provided with leaf springs 214, 219 attached thereto with screws. The leaf springs 214, 219 are pressed against the shafts 202, 203, respectively. When the lens barrels 201, 225 move, therefore, they slide frictionally along the shafts 202, 203. Piezoelectric elements 212, 217 are attached to the respective rear end portions of the shafts 202, 203. The piezoelectric elements 212, 217 have respective rear end portions secured to the upright portions 213e, 218e of the support members 213, 218. The support members 213, 218 are secured to a common fixed portion.
The principle of operation of a moving mechanism using piezoelectric elements is disclosed in U.S. Pat. No. 5,225,941 or the like, which will be described briefly with reference to the schematic diagram shown in FIG. 2. As shown in (a) section of FIG. 2, a shaft 53 is secured to one end of a piezoelectric element 52, while a fixed portion 51 is secured to the other end thereof. A moving member 54 to be moved is held on the shaft 53 under a frictional force produced by spring biasing. The mass of the fixed portion 51 is sufficiently large compared with the mass of the moving member 54.
When a voltage is applied to the piezoelectric element 52, the inertial force of the fixed portion 51 prevents the piezoelectric element 52 from extending in the direction of the fixed portion 51, so that the piezoelectric element 52 extends in the direction of the shaft 53 to move the shaft 53 to the left in the drawing. At this time, on a gentle rising edge of the applied voltage as shown in A section of FIG. 3, the moving member 54 moves by a distance x along with the shaft 53, as shown in (b) section of FIG. 2, since the frictional force produced between the moving member 54 and the shaft 53 is larger than the inertial force of the moving member 54.
Next, when the piezoelectric element 52 releases a voltage on a steep falling edge as shown in B section of FIG. 3, the inertial force of the moving member 54 becomes larger than the frictional force produced between the moving member 54 and the shaft 53, so that the moving member 54 remains in place and only the shaft 53 moves by the distance x toward the initial position, as shown in (c) section of FIG. 2. By applying a sawtooth drive pulse to the piezoelectric element 52 to cause it to repeat this action, the moving member 54 can be moved to a specified position. To move the moving member 54 in the opposite direction, a drive pulse with a steep rising edge and with a gentle falling edge is applied properly.
If the frequency of a waveform as shown in FIG. 3 is increased to increase a frequency for extending and contracting an electro-mechanical transducer, a transition is made to a state in which a skid occurs between the driving frictional member (shaft) and the moving member which are frictionally combined with each other upon each of the extension and contraction of the electro-mechanical transducer. In this case, since a relative skid occurring between the driving frictional member and the moving member upon the extension of the electro-mechanical transducer is different in direction and magnitude from the skid occurring therebetween upon the contraction of the electro-mechanical transducer, the moving member can be driven in a desired direction relative to the fixed portion.
In the lens driving apparatus having a structure as shown in FIG. 11, the piezoelectric elements 212, 217 extend and contract when a sawtooth drive pulse is applied thereto for a specified period of time, thereby causing the forward and backward movements of the shafts 202, 204. The order relationship between the frictional force between the shafts 202, 203 and the leaf springs 214, 219 and the inertial force resulting from the masses of the lenses and the lens barrels 201, 225 vary depending on the moving speeds of the shafts 202, 203, so that the lenses are moved to specified positions. It is to be noted that the force applied to the fixed portion upon the extension or contraction of the piezoelectric elements 212, 217 causes the fixed portion to reciprocate along with the support members 213, 218, though it is indistinct because of the relatively large mass of the fixed portion.
In FIG. 4, the vertical axis represents the magnitude of a voltage and the horizontal axis represents time. In the case where the drive pulses applied to the two piezoelectric elements 212, 217 have waveforms G1, G2 shown in FIG. 4 in which portions with abrupt voltage changes B1, B2 are coincident with each other, the forces are applied simultaneously to the fixed portion so that the respective driving operations interfere with each other.
For example, the fixed portion experiences an increased amount of elastic deformation and an increased amount of travel compared with the case where a single piezoelectric element and a single shaft are provided, so that the shafts 202, 203 (driving shafts) undergo a lower degree of acceleration.
occasionally, the inertial forces of the lens barrels 201, 225 (moving members) become smaller than the frictional forces, which deteriorates driving properties. As a result, the speeds of the lens barrels 201, 225 are lowered or, in some cases, the lens barrels 201, 225 are not moved at all. This leads to such a problem as an elongated time required to dispose the lenses at desired positions, which degrades the operability of a camera used in conjunction with the lens unit. Similar problems occur when three or more piezoelectric elements are used.
Although the description has thus been given to the case where the common fixed portion is used, interference occurs even when fixed portions are provided for individual piezoelectric elements on a one-by-one basis if the fixed portions are at positions close to each other. That is, if the voltages applied to the piezoelectric elements have the waveforms G1, G2 in which the portions B1, B2 with abrupt voltage changes are coincident with each other, as shown in FIG. 4, the forces are applied simultaneously to the fixed portions and therefore the forces are applied simultaneously to the lens unit in which the fixed portions are disposed.
This also deteriorates the driving properties for the lens barrels (moving members), similarly to the foregoing case. If the fixed portions contain elastic bodies such as plastic moldings, in particular, the fixed portions are likely to warp and move, so that the amount of elastic deformation experienced by the fixed portions is increased and the influence given to the driving of the moving members becomes greater.